1. Field of the Invention
The present invention relates to the treatment of aqueous suspensions of particulate solid material which comprise kaolin. Additionally, the present invention relates to the recovery and use of solids-containing products produced by the process.
Many naturally occurring mineral materials are subjected to processing steps such as particle size reduction and separation in order to select those particles which have the most desirable distribution of sizes for a particular application. In many cases the natural mineral material or the material obtained by processing, eg grinding, contains a significant proportion of particles which are undesirable for the particular application for which the mineral material is being prepared, and it is necessary to remove these excessively fine particles. When the particle size separation is performed on a mineral material in suspension in a liquid, which would most commonly be water, and when the undesired fine particles have an equivalent spherical diameter of about 1 .mu.m or less, especially 0.5 .mu.m or less, the suspension of the undesired fine particles is often obtained in the form of a dilute suspension which is extremely difficult to dewater by conventional methods. It is generally unacceptable, for environmental reasons, to allow such dilute suspensions of fine mineral particles to be discharged to rivers or lakes, and, as a result, such unwanted suspensions of very fine particles are often retained in lagoons, thus occupying large areas of land which could more profitably be used for other purposes.
Kaolin is a particulate inorganic solid material which finds wide use as a pigment, filler, extender or the like in a variety of applications, eg paper filling and coatings, polymers, plastics, paints, sealants and the like. Kaolin is generally obtained from mineral sources and is treated by a series of processing steps which provide refinement to provide suitable product properties. The treatment may include particle size reduction and/or separation steps as referred to earlier. Such treatment has been practised industrially for many years.
One purpose of the present invention is to provide a method by which properties of a particulate kaolin product may be improved.
Another purpose of the present invention is to provide a method by which a suspension comprising very fine kaolin, separated from a main kaolin product by a particle size separation process, and hitherto regarded as a waste material, may be recovered and re-used.
The present invention, as described hereinafter, is concerned with forming an inorganic precipitate in an aqueous medium containing suspended kaolin particles from a kaolin processing operation, which in some instances may comprise kaolin particles previously regarded as waste because of their fine nature, and thereby forming a useful composite product with the kaolin which can be re-used for example in paper making. Such a process is not suggested in the prior art.
2. Summary of the Prior Art
A chemical process for dealing with the solid material in a paper plant effluent stream is described by O'Brien in U.S. Pat. No. 4,115,188. The solids present are coagulated and flocculated by changes in pH.
Formation of a precipitate in a waste aqueous medium is known per se. For example, such a process is described by Le Compte in U.S. Pat. No. 3,262,877, Spruill in U.S. Pat. No. 3,639,206 and Opferkuch in U.S. Pat. No. 4,076,620 but in these cases there is no disclosure or suggestion of precipitating specifically onto suspended particles and making use in paper making or other processes of the product formed. It is to be noted that Spruill uses lime to provide a flocculating effect on suspended solid material in an effluent stream from the pulping of wood. However, the suspended lime-treated solid material is separated by a clarification step. Although precipitation is subsequently carried out using addition of carbon dioxide the treated aqueous medium contains substantially no suspended solids when the precipitate is formed. Thus, no composite is formed as in the method of the present invention. The waste water which is produced in the various stages of the process described by Spruill is not suitable for treatment to recover a useful composite product therefrom because it is too dilute.